The interest which has recently been shown in using solar radiation as an industrial source of energy is very well known indeed. It is not unreasonable to affirm that at present the main problems have already been resolved in the design of an electrical energy generating plant making use of energy radiated by the sun in order to heat one or more fluids, to produce vapor and to then drive a turbo alternator. The investment costs involved with the provision of a heliothermic generating plant are also known.
A very large part of this investment is tied up with the provision of heliostats arranged over a large area. These pieces of apparatus, which are fixed to the ground, receive solar energy and reflect it onto a receiver, which heats up a fluid.
Various types of heliostats do exist of which one is the so-called momopode.
This apparatus consists of a leg or column, on the plane top of which a head revolves which houses the revolving mechanism, which is also referred to as the azimuth mechanism. The head has fixed to it, in a manner which allows for rotation, a supporting structure for one or more panels of reflecting mirror; and which, furthermore, is provided with its own mechanism, referred to as the elevating mechanism, which provides for the above described rotary motions. The revolving mechanism for the head operates about the vertical axis of the column; whilst the elevating mechanism of the supporting structure for the reflecting panels operates about a horizontal axis, which is normal to the vertical axis referred to.
A large area array of heliostats, comprising many thousands of heliostats, is exposed to the direct action of natural ecological factors, these being: direct solar radiation, temperature changes; rain, wind, hail, and the like.
The task of an elevating mechanism is not limited to that of following the sun, this of course being an extremely important task, but furthermore it must position the reflecting panels horizontally, with the mirrors directed downwardly, in order to protect them in the case of a hail storm.
It is also necessary for them to take up this position in order to provide the greatest margin of security when they are subject to the direct action of strong winds, of hurricane force; although in this case it is possible for the mirrors to remain with their surfaces directed upwardly.
Since the investment costs involved with a large area array of heliostats may constitute something of the order of fifty percent of the total investment costs of the generating station, it is of prime importance not only to bring these costs down, but also to reduce as much as possible expenses for maintenance and upkeep. It is consequently vital to provide not only standardized large scale production of the various components of heliostats; but also to employ a reduced number of these as well as providing the possibility of carrying out the maximum number of operations using one individual machine tool and, clearly, as well, it is necessary to eliminate on-site assembly operations as much as possible by providing the maximum number of pre-assembled parts to be delivered in units to the site. It is also important to provide an operating mechanism which uses a reduced number of parts, and these should be as simple as possible: this being done in order to reduce maintenance and upkeep costs so that each heliostat is as durable as possible.
In order to provide a heliostat with a long working life, and in the case of present day projects this is expected to be not less than thirty years, the heliostat must be designed to withstand any occurence which might reasonably be expected. For example it must withstand the direct action of a hurricane. In order to withstand the effects of a hurricane, the heliostats must be arranged so that their panels are in a horizontal position, but should one of them not comply with the order, as a result of a breakdown, it would put the whole of the large area array of heliostats in grave danger since should one of these or a part of one be torn from its mountings, the wind could use it as a projectile against the others.
Nevertheless, the designing of heliostats in order that they can withstand the action of hurricanes, when their maximum surface is exposed to these, is an expensive solution.
From another point of view, the energy yields, and the economic gain, from a heliostat generating plant depends essentially on the degree of precision of the focusing of the panels of mirrors on the receptor. In the case of projects which are actually being worked on there is a requirement with all designs that the focusing error of a heliostat should not be greater than two milliradians. It should be born in mind that only the best structural foundations of the supporting columns can provide a figure which is on the order of one milliradian. Furthermore, the inevitable yielding of every supporting structure is present, the structure consisting for example of the column itself, the structure which carries the reflecting panels in an overhanging manner, which themselves act as plates which are supported at three of four points. For this reason, it is just as important to reduce, as much as is possible, play in the mechanism, by using parts which are more suitable and reducing the number of individual components comprising these.
Additionally, in order to achieve the necessary thirty years working life it is necessary to adopt, on the one hand, mechanisms which are subject to the least possible wear, using larger surface contact areas, and reduced contact pressures; and on the other hand to reduce to a minimum, or even to avoid, all types of residual stress which decreases the stress resistance of the materials used.
The reduction of residual stresses is vital for the survival of a mirror, the limits of endurance of which are per se somewhat low. It is very important to avoid tractional forces on a mirror. Generally these result either from the technological processes used in shaping the mirror, or from thermal effects, since the reflecting panel is made up of materials having very differing coefficients of thermal expansion.
For all these reasons, in the case of the best present day designs of heliostats, the use of curved mirrors has been dropped, and this has given way to the use of plane panels, to the detriment of the energy yield of the generating unit. In this same connection, this indicates that the use of aluminum should also be dropped, this being a material having a high coefficient of expansion, for the support for the mirrors.
However, when attempts have been made to obtain concave surfaces, using support materials such as foam of various plastics, polyester or polyurethane, using machining of these, it has not been possible to obtain a geometry which meets the necessary tolerance requirements. This is another of the reasons which made it necessary, in the case of the designs referred to, to drop the use of curved mirrors. Designs in which the forced deformation of the mirror was decided upon as a solution, either using threaded tensioners or using the permanent action of a vacuum, have met with failure because the concave geometry was obtained by introducing tractional stresses, bending stresses and membrane stresses which sharply reduce the fatigue resistance of the mirror.
Finally, it should be pointed out that the energy yield from a heliostat generating plant is very closely tied up with the consumption of energy used by the heliostat for operating their mechanisms which follow the sun. In its turn, the power of the mechanisms referred to is a direct function of the lever arm of the couple providing the turning force: the larger the arm, the lesser the power, for the same turning moment.
As has already been said, the elevating mechanism should be able to provide for the reflecting panels to turn through half a turn of 180.degree.. In present day designs, this is obtained by means of the rotation of a tube which is the principle component of the supporting structure for the reflecting panels, and which rests onto two large diameter bearings which surround it and allow it to rotate. Rotation of the above said tube is obtained, either using a gear wheel, which is rigidly fixed to the tube, and a reduction gear associated with the motor providing elevation; or is obtained using two linear actuators, one of which together with the tube referred to and an additional lever which pivots on the same bearing as the tube, constitutes a triangle having a variable configuration. Both solutions, in addition to requiring expensive bearings of large diameter or a combination of equivalent swivel joints, suffer from the disadvantage of having short lever arms which require a greater power.